Process for the separation of air



June 18, 1963 M. ARMSTRONG ETAL 3,0 0

PROCESS FOR THE SEPARATION OF AIR Filed Oct. 17, 1961 WASTE A I R NITROGEN TO COLUMN INVENTORS KEANE TH CECIL 5 1(7)? J Mlcrmfl 412113710110 A'F ORNEY 3,094,402 PRQCESS FDR THE SEPARATION OF AlR hiichael Armstrong, Chertsey, and Kenneth Cecil Smith,

Carshalton, England, assignors to The British ()xygen Company Limited, a British company Filed Oct. 17, 1961, Ser. No. 145,560 Claims priority, application Great Britain Get. 17, 1%!) 6 Claims. (Cl. 62-9) This invention relates to the separation of air by liquefaction and subsequent rectification.

The separation of air into oxygen and nitrogen to produce a substantial proportion (for example, about 20%) of the oxygen output in the liquid phase is usually carried out in plants in which the air is cooled while compressed and then expanded so that a considerable degree of liquefaction takes place, the liquefied air then being rectified in a rectification zone. Various methods are available by which the separation can be carried out at fair thermodynamic efi'iciency, one of these being the Heylandt cycle.

In this cycle, air compressed to a relatively high pressure of about 170 atma. is precooled by heat exchange with a separated gaseous nitrogen fraction and is then divided into two stream, a major stream consisting of about 60% of the air and a minor stream consisting of the remaining 40%. The minor stream is cooled by passage through a main heat exchanger countercurrent to the separated gaseous nitrogen fraction on its way to the precooler, and is then expanded to the rectification pressure of about atma. through an expansion valve. The major stream is expanded to the rectification pressure through an expansion engine with the performance of external work. The two streams are then re-combined and the combined stream fed to the rectification Zone. The Heylandt cycle, by recovering some of the energy present in the compressed gas, saves a certain amount of power and is therefore more eficient than cycles in which no Work is performed by the gas.

In the Heylandt cycle, it is impossible to operate the main heat exchanger in such a manner that the theoretical maximum quantity of energy is recovered. It has been calculated that for maximum efliciency the temperature difference between the streams in heat exchange should be zero everywhere in the heat exchange It is impossible in practice to achieve this end, with the result that a certain amount of refrigeration is Wasted.

It is an object of this invention to provide a modification of the Heylandt cycle in which use is made of some of this wasted refrigeration with a consequent reduction in power consumption.

According to the invention, a process for the separation of air by liquefaction and subsequent rectification comprises precooling air compressed to a relatively high pressure by heat exchange with a gaseous nitrogen product, dividing the precooled air into a major stream and a minor stream, cooling the minor stream in a main heat exchanger by heat exchange with the gaseous nitrogen product passing to the precooling step, expanding the cooled minor stream to rectification pressure, expanding the major stream to an intermediate pressure in an expansion machine with the performance of external work, warming a minor part of the expanded major stream by passing it through an intermediate section of the main heat exchanger countercurrent to compressed air passing through the heat exchanger, combining the warmed part with the remainder of the major stream, further expanding the re-united major stream to the rectification pressure in an expansion machine with the performance of external work, and subjecting the expanded major and minor streams to rectification in a rectification zone. Pref- States Patent G 3,094,402 Patented June 18, 1963 erably, the expanded major and minor streams are recom bined prior to rectification.

The relatively high pressure to which the air is initially compressed will usually be about 150 atma., the rectification pressure about 5 atma. and the intermediate pressure between about 8 and 50 atma., for example, about 18 atma. The major stream will usually comprise about 60% of the total air and the minor stream remaining 40%. The optimum quantity of the major stream to be warmed in the main heat exchanger Will depend on the intermediate pressure used. For an intermediate pressure of 18 atma., about 18% should be so warmed.

One example of the process of the invention will now be described in more detail with reference to the accompanying drawing which shows diagrammatically a flow sheet of the process.

volumes of air compressed to atma. and cooled to 280 K. by any suitable means (not shown) are passed through a precooler 1 where they are cooled to a temperature of 250 K. by heat exchange with a gaseous nitrogen product.

The air stream leaving the precooler 1 is split, 38.7 volumes being passed through a main heat exchanger 2 as hereinafter described. The remaining 61.3 volumes of the air are passed through an expansion engine 3 where they are expanded to a pressure of 18 atma. with the performance of external work. The temperature of the air leaving the expansion engine 3 is 144.3" K. 10.94 volumes of the expanded air leaving the engine 3 are passed through a separate path in the main heat exchanger 2 countercurrent to the compressed air stream. This path does not pass completely through the exchanger 2 but is limited to an intermediate central section thereof. The temperature of the air sub-stream passed through this path rises from 144.3 K. at point F to 182.0 K. at point E. This war-med air stream is then recombined with the remainder of the air leaving the expansion engine 3, the temperature of the combined stream being 150.7 K. The combined stream is passed through a second expansion engine 4, where its pressure is reduced to the rectification pressure of 5.5 atma., and its temperature to 110.4 K.

The 38.7 volumes of the air which enter the main heat exchanger 2 at a temperature of 250 K. emerge from it at a temperature of 97.7 K., the pressure still being 150 atma. The intermediate temperature at points M and N (corresponding to points E and F in the reheat section) are l95.5 K. and 153.3 K. respectively. After leaving the heat exchanger 2, this air stream is expanded to 5.5 atma. through an expansion valve 5.

The air stream leaving the valve 5 and that leaving the expansion engine 4 are then combined, and the combined stream consisting of a mixture of liquid and vapour, is then passed to a conventional separation column (not shown) wherein it is separated to obtain 19.38 volumes of liquid oxygen supercooled by 1l.9 K., 0.42 volume of liquid argon and 80.2 volumes of gaseous waste nitrogen (i.e. nitrogen containing small amounts of oxygen and argon and very small amounts of other inert gases).

The waste nitrogen at a temperature of 84.4 K. and at atmospheric pressure is passed through the main exchanger 2 countercurrent to the compressed air stream, leaving the exchanger at a temperature of 228.2 K. The temperature at intermediate points Q and R, corresponding to points N and F and to points M and B respectively are 144.3 K. and 187.1 K. respectively. The waste nitrogen is then passed through the precooler 1 where its temperature rises to 278 K.

The power consumption of the process will depend to some extent on the efiiciency of the expansion engines 3 and 4 but it would be impossible to obtain the same amounts of supercooled liquid oyxgen and liquid argon from a conventional Heylandt cycle with a starting pressure of only 150 atma.

I claim:

1. Process for "the separation of air by liquefaction and subsequent rectification comprising preoooling air compressed to a relatively high pressure by heat exchange with a gaseous nitrogen product, dividing the precooled air into a major stream and a minor stream, cooling said minor stream in a main heat exchanger by heat exchange with the gaseous nitrogen product passing to the precooling step, expanding the cooled minor stream to the rectification pressure, expanding said major stream to an intermediate pressure in an expansion machine with the performance of external work, warming a minor part a minor streams to rectification in a rectification zone.

2. Process according to claim 1 wherein said expanded major and minor streams are combined prior to rectification.

3. Process according to claim 1 wherein the major stream comprises about by volume of the total air.

4. Process according to claim 1 wherein the relatively high pressure to which the air is initially compressed is about atrna., the rectification pressure is about 5 atma. and the intermediate pressure is between about 8 and 50 atma.

5. Process according to claim 4 wherein the intermediate pressure is about 18 atma.

6. Process according to claim 5 wherein the amount of the major stream warmed in the main heat exchanger is about 18% by volume.

References Cited in the file of this patent UNITED sTATEs PATENTS Re. 19,267 Van Nuys Aug. 7, 1934 967,104 Claude Aug. 9, 1910 1,901,389 Hazard-Plamand Mar. 14, 1933 2,078,953 Levin May 4, 1937 2,645,103 Fausek July 14, 1953 FOREIGN PATENTS 721,841 Germany June 19', 1942 826,298 Great Britain Dec. 31, 1959 831,613 Great Britain Mar. 30, 1960 

1. PROCESS FOR THE SEPARATION OF AIR BY LIQUEFACTION AND SUBSEQUENT RECTIFICATION COMPRISING PRECOOLING AIR COMPRESSED TO A RELATIVELY HIGH PRESSURE BY HEAT EXCHANGE WITH A GASEOUS NITROGEN PRODUCT, DIVIDING THE PRECOOLED AIR INTO A MAJOR STREAM AND A MINOR STREAM, COOLING SAID MINOR STREAM IN A MAIN HEAT EXCHANGER BY HEAT EXCHANGE WITH THE GASEOUS NITROGEN PRODUCT PASSING TO THE PRECOOLING STEP, EXPANDING THE COOLED MINOR STREAM TO THE RECTIFICATION PRESSURE, EXPANDING SAID MAJOR STREAM TO AN INTERMEDIATE PRESSURE IN AN EXPANSION MACHINE WITH THE PERFORMANCE OF EXTERNAL WORK, WARMING A MINOR PART OF THE EXPANDED MAJOR STREAM BY PASSING THROUGH AN INTERMEDIATE SECTION OF THE MAIN HEAT EXCHANGER COUNTER- 